Absolutely nothing!
Ok not strictly true but I hate the sight of pens adorned with all all kinds of litter!!!!!
Firstly, make sure the habitat in your pen is right. 1/3 ground cover is the GWCT recommendation and can be anything from a bracken and nettles through to laurels, lonicera, dead hedging and brash or even laid elder and Hazel or just some good thick long grass.
Secondly, think about where you place feeders and how many you have. Week one, I have one hopper per fifty poults and place them along the edges of all the cover so the poults can feed quickly and safely. At least half of these should be Mannolas as they will recognise them and be able to shelter from buzzards under the rim of the lid.
Thirdly, find a supplier that can supply you with fit, strong, alert and well feathered poults. This is most important I'd say. It's also more important than size/age. I used to get more 8/9week old poults taken at my old shoot than I do 6.5/7 week old poults where I am now because they're being released into much nicer pens in terms of habitat.
I had my first poults this week on Thursday and despite having a pair of buzzards in all three woods, I've only lost 1 poult out of 6000! And that was on day one and was a bit of a runt anyway.
I do leave a lot of dead rabbits on the maize plots this time of year too to try and make sure the buzzards all have full crops!!